The present invention relates generally to digital image processing and, more particularly, to a system and method thereto where a color is represented by an unique set of values in a first color space corresponding to more than one set of values which represent the same color in a second color space, and where in optimization regime is utilized to choose one particular set of values from the second color space to be associated with the set of values from the first color space. In a particularly useful embodiment where a printer's CMYK digit space is considered the second color space and the L*a*b* domain of printed CMYK combinations is considered the first color space, the invention will provide a singular set of CMYK values determined from all CMYK combinations which produce the same color represented by an unique set of L*a*b* values. By utilizing the invention to produce unique correspondences between a useful sampling of the first color space and sets of values from the second color space, color profiles for use in color management environments are developed.
Digital image processing involves electronically capturing a color image of a scene, altering or otherwise processing the captured image in some desired fashion and passing the processed image to a rendering device. The processing and transfer of a color image throughout a digital image processing system requires the color components of each pixel of the captured image to be represented in various device-dependent color spaces. An upstream element of a digital image processing system can be thought of as a source device associated with a source device color space for representing pixel colors of an input image, whereas a downstream element can be thought of as a destination device associated with a destination device color space for rendering an output image. For instance, a simple image processing system could include: an image acquisition device such as a digital camera, camcorder, scanner, CCD, etc.; a color processor for processing the colors of the image; and an image rendering device, such as a printer, monitor, computer memory, etc. When considering a communication between the image acquisition device and the image rendering device, the image acquisition device is deemed as the source device whereas the image rendering device is deemed as the destination device.
Moreover, image processing can extend beyond the simple systems noted above to special use systems such as a system for proofing images which would include a pre-press proofer, a processing module and a printing press.
Transferring images and documents between digital imaging devices such as monitors, scanners, printers, pre-press proofers and printing presses requires color matching, i.e. matching of color characteristics of the respective devices since different imaging devices have different color capabilities, describe color characteristics in different terms, and operate in different color spaces. For example, a color display monitor in a computer system may operate in RGB color space by creating and describing colors in terms of red, green and blue (RGB) values. The RGB values associated with particular colors for the display monitor are device-dependent in that the RGB values associated with specific colors are particular for the given monitor. Since the RGB values are device-dependent, colors displayed on different monitors will probably not be visually identical even for the same RGB input values.
In contrast to monitors, most printers use cyan magenta, yellow and black (CMYK) values to describe colors, and are said to operate in the CMYK color space. Since the CMYK values are also device-dependent, colors printed on any given printer will probably not match colors printed on a different printer for the same CMYK value.
Different devices also have different color capabilities. Every rendering device, such as a printer or monitor, has a limited range of colors, i.e. a gamut, that it can reproduce. Those skilled in the art will recognize that color display monitors tend to be able to produce a wider range of lighter colors whereas color printers tend to be able to produce a wider range of darker colors. Consequently, the gamut for a color display monitor is different from the gamut for a color printer. As a result, some colors displayed on display monitors cannot be reproduced on color printers and vice versa.
In recent years, device-independent paradigms for the characterization of color information in an image processing system have been developed and are being implemented. ColorSync, developed by Apple Computer and KCMS, developed by Eastman Kodak Co., are examples of systems or components supporting a device-independent color paradigm. This paradigm is based upon a characterization of the image pixel data or digits in a device-independent color space, e.g. CIE L*a*b* or CIE XYZ, using a color management system. Device digits are defined as independently specifiable pixel values which control how a device renders color. For example, a four color printer would print each pixel having specific device digits corresponding to the CMYK values.
The characterization of a device's image pixel data in device independent color space is commonly codified in a tagged file structure, referred to as a device profile, that accompanies the digital imaging device. A device profile is a digital representation of the relation between device coordinates and a device-independent specification of color. Other types of profiles are device link, color space conversion, abstract or named color profiles. Device link profiles provide a mechanism in which to save and store a series of device profiles and non-device profiles in a concatenated format as long as the series begins and ends with a device profile. Profiles are standardized and further discussed in the International Color Consortium (ICC) Profile Format Specification, Version 3.3, Nov. 11, 1996, page 101. Moreover, non-color spatial information can be included in profiles as disclosed in U.S. patent application Ser. No. 08/709,487 filed Sep. 6, 1996 by Hultgren et al. Both documents are herein incorporated by reference in their entirety for supplemental background information which is non-essential but helpful in appreciating the applications of the present invention.
When transferring color information from a 3 dimensional color space, such as L*a*b*, to a 4 dimensional color space, such as CMYK, a one-to-many problem occurs. In other words for an unique set of L*a*b* values, more than one solution set of CMYK values may be available. This problem is well recognized in the industry and addressed by known methods such as under color removal (UCR), gray component replacement (GCR) and under color addition (UCA). These methods and similar processes have been developed by skilled operators with years of crafts experience implemented by a first conversion to the three dimensional color space of CMY and followed by the introduction of the black component (K) in place of some amount of CMY. The final levels of K and CMY are dependent upon the CMY values resultant from the first conversion.
It is a primary object of the present invention to provide a system and method thereto for solving the one-to-many problem described above. This and other objects and advantages of the invention will be apparent from the following detailed description.